Avoiding vaporisation (cavitation) at siphon summits To avoid vaporisation in a pipeline crossing a ridge (i.e., at a siphon summit), the pipe centerline should be kept above the hydraulic gradient line by not more than approximately:

Introduction / Context:
At high points in pressure pipelines (siphon summits), the static pressure may drop. If it falls below the vapor pressure head, vapor pockets form (cavitation/air binding), disrupting flow and damaging equipment. Designers therefore limit how far the summit rises above the hydraulic grade line (HGL).

Given Data / Assumptions:

• Water at ordinary ambient temperatures.
• Allowance for minor losses and fluctuations.
• Rule-of-thumb limits applied in water-supply engineering.

Concept / Approach:
The vapor pressure head of water at room temperature corresponds to roughly 2–3 m of water column relative to absolute vacuum. Considering safety margins and dynamic effects, practice limits the summit’s elevation above the HGL to on the order of 6 m (often quoted around 6.0–6.5 m) to prevent local pressures from dropping to vapor pressure.

Step-by-Step Solution:

Verification / Alternative check:
More conservative designs choose ~6 m; the provided options include 6.4 m, aligning with typical textbook values.

Why Other Options Are Wrong:

• 2.4 m: Too conservative; unnecessary for most systems.
• 10.0 m: Risks cavitation for ordinary temperatures.
• 5.0 m: Possible but not the standard value usually cited in exam contexts.

Common Pitfalls:
Ignoring temperature dependence of vapor pressure; neglecting transient (water hammer) depressions that can further reduce pressure at the summit.

Final Answer:
6.4 m